Bill was Hattori Hanzo's former student. He used Hanzo's teachings for dishonorable purposes, causing Hanzo to retire.
But when Beatrix shows that she has the resolve to defeat Bill, Hanzo feels honorbound to make her a sword so she'd have a chance at winning.
From ZacPensol at reddit
Considering that everyone seemed to make a big deal out of Beatrix having a Hanzo sword, and the emphasis put on Budd having one, I don't think that the rest of the team had them.
Beatrix refers to Bill as a former pupil of Hanzo's, and Hanzo knows exactly who she's referring to. I always figured that Bill studied swordplay under Hanzo in a similar way that he studied martial arts from Pai Mei. However, Hanzo was perhaps put off by Bill's darker tendencies because Bill saw the swordsmanship as a means of creating violence instead of ending it.
So whatever happened and Bill quit being Hanzo's apprentice and went on to form the DiVAS. Hanzo is a guy with connections most-certainly. I'm sure it's no stretch of the imagination that he eventually heard about - or even kept tabs on - Bill and how he'd become the leader of such a notorious assassination squad. His shame at having aided in creating not only weapons that kill people, but "creating" Bill as a deadly individual, lead him to retire in guilt so as not to ever create something or someone as horrible as Bill again.
He, of course, breaks this oath when Beatrix shows up because, though he's hesitant to create another great sword that could so easily kill more people, in Beatrix he not only sees a lightness that was not in Bill, but a way to secure his heart and mind that the evil he'd helped create "Bill" would be brought to an end. Perhaps this wouldn't wipe free his guilt over Bill's former actions, but it would at least ease his mind to know that Bill's reign of terror was over.